1. Field of The Invention
This invention relates to a system for monitoring the transmission capability of power transmission lines. More particularly the invention relates to an apparatus and method for determining the current carrying capacity of power lines which is limited by the allowable sag of the line between transmission towers. To control the load on a power line it is necessary to know the condition of the line in terms of the sags of the different spans. The present invention may incorporate the method and apparatus set forth in applicant's U.S. Pat. No. 5,235,861 entitled Power Transmission Line Monitoring System the disclosure of which is herewith incorporated by reference.
2. Description of the Prior Art
Certain problems associated with overhead power transmission lines regarding sag and clearance between the line and the nearest point on earth, are set forth in applicant's above cited prior patent. This patent also outlines prior art solutions to this problem and cites certain prior patents. The system for measuring power line sag between spans described in the said patent employs a novel tension monitoring system in which the line tension of the power line is measured and appropriate calculations are made as outlined in the patent in order to determine sag by which the person controlling the power line can appropriately adjust the load.
The tension measuring load cells will also detect other line conditions such as icing, annealing, galloping, creep, wind loads and other phenomena effecting line tension.
The tension measuring unit is conventionally a load cell including strain gages interposed in the power line span adjacent to a tower, that continuously measures the tension on the line. The strain gages are generally connected in the form of a Wheatstone bridge with two legs under tension and two under compression. An electrical signal is produced that may continually monitor the tension on the power line. This signal is transmitted over a cable to a remote electrical processing system located in an instrumentation console as shown and described in the applicant's prior patent.
The present state of the art has two significant problems. These are difficulties in the protection of load cells at locations where some circuits are energized during installation and maintenance and difficulties in protection of the load cells against electric overvoltages caused by lightning and other circuit transients.
Thus when a load cell is installed at a structure with energized circuits, the load cell is subject to electrical damage, because during the installation and maintenance, the cell cabling will be in a high voltage environment without a ground shield.
The second danger factor is that of atmospheric overvoltages and transients in the high voltage transmission circuit due to lightning or other causes. Because a load cell body is electrically connected to the structure, the body is at the same potential as that of the location of its attachment, while the strain gages forming the bridge elements of the load cell are connected to and have the same potential as the measuring equipment. Because the load cell bodies and the measuring equipment are joined by a common ground wire (along the transmission structure) they remain at the same potential under normal 60 Hz operating conditions.
It is to the solution of these problems that the present invention is directed.